Reflectometry consists in transmitting a signal over a cable network and in then measuring the returned signals. The delay and amplitude of these echoes enable information to be obtained about the structure or electrical faults present in said network. More generally speaking, each echo corresponds to one singularity. It is then possible to locate, characterise and possibly predict a breakdown. The well-known conventional reflectometry methods are TDR (Time Domain Reflectometry) and FDR (Frequency Domain Reflectometry).
The purpose of reflectometry is to retrace the abrupt variations in the characteristic impedance of one or more transmission lines, which may correspond to branches or faults (short circuit or open circuit), by determining the reflection coefficients αk of the line or lines.
The principle of reflectometry is based on the propagation of voltage waves in these lines. It consists in sending a known signal x(t) in an electrical line and in then measuring the signal xs(t) returned by the line. The measurement taken then consists of the set of echoes due to the variations in the characteristic impedance of the line. Analysis of the measurement xs(t) in relation to the original signal x(t) provides an estimate of the line response.
The diagram of FIG. 1 shows the overall operation of a reflectometry device. This figure shows:                a transmission portion which includes:                    a shape generator 10,            a digital-to-analog converter 11,            a first electrical coupling device 12 connected to the line being diagnosed 13,                        a reception portion which includes:                    a second electrical coupling device 14 connected to line 13,            an analog-to-digital converter 15,            a signal averaging and processing device 16.                        
The signal x(t) sent on line 13 by the transmission portion is subject to constraints specific to the application and reflectometry method used. These constraints result from the interference problems between the reflectometry signal and the target signal. The electrical coupling is a purely electrical portion the function of which is to carry out the interfacing between the reflectometry device (transmission, reception of the signal) and the line 13 by providing functions such as impedance matching, electrical protection or else possible filtering.
The relationship between xs(t) and x(t) is given by the relationship: xs(t)=x(t)*h(t), * representing convolution product. In the frequency domain, the following relationship is obtained: Xs(f)=X(f)H(f).
As a first approximation, it may be considered that h(t) is a series of Dirac pulses corresponding to the expression:h(t)=Σαkδ(t−τk)  (1)
In diagnostic systems, reflectometry is generally carried out by a digital system. Sampled signals are then worked on, and the final result, namely the pulse response, is likewise sampled (or discretised).
The test signal is defined by the vector s=(s0, s1, . . . sN-1), N being the number of samples corresponding to one period. This signal may or may not be transmitted periodically, based on the type of signal and the processing carried out by the system. When the transmission is periodic of period N, the measurement signal obtained after sampling is given byŷ=Hs+n  (2)
y=(y0, y1, . . . yN-1): discretised measurement signal,
n represents the additive measurement noise.
H is the matrix M×N corresponding to the pulse response of the discretised line.
  H  ⁡      (                                        h            0                                                h            1                                    …                                      h                          N              -              1                                                                        h                          N              -              1                                                            h            0                                    …                                      h                          N              -              1                                                            ⋮                          ⋱                          ⋱                          ⋮                                                  h            1                                                h            2                                    …                                      h            0                                )  
This matrix H represents the convolution operation. Post-processing of the signal ŷ next enables an estimate of the line pulse, response to be retrieved, which can be analysed in order to detect possible faults.
In practice, it is often advantageous to carry out several measurements and to calculate the average thereof so as to improve the signal-to-noise ratio. Since the test signal is indeed transmitted periodically, if it is considered that the line response h varies slowly relative to the period T=NTe, it may be considered that the signal remains stationary over several measurements. The final measurement signal is then obtained by
                                          y            ^                    _                =                              1            M                    ⁢                                    ∑                              m                =                0                                            M                -                1                                      ⁢                                          y                _                                            (                m                )                                                                        (        3        )            
M: is the number of measurements
y(m): is the mth measurement.
The objective of distributed reflectometry is to be capable of carrying out several reflectometric measurements simultaneously at several points of a wired network in order to resolve the topology-related ambiguities.
Distributed reflectometry enables diagnostic measurements to be carried out simultaneously at several points of a network, several reflectometers being connected to said network and transmitting a test signal at the same time. The problem solved by this method is that of interference with the measurement of each reflectometer by the test signals of the other reflectometers, which requires processing of the signal received. Distributed reflectometry thus enables certain ambiguities related to the structure of the network to be resolved. To illustrate this problem of ambiguity, reference may be made to the case of a Y network. When a fault is detected in such a network, the distance separating the same from the point of measurement is known, however it is not always possible to determine the branch on which the same is located. Among other things, distributed reflectometry enables this type of ambiguity to be resolved, by carrying out processing which consists in discriminating between the signals derived from the various reflectometers so as to preserve only the signal of interest. A discrimination algorithm is described as the processing which enables this functionality to be achieved.
As shown in FIG. 2, p reflectometers are thus connected to a network 20 and transmit a test signal at the same time. Each reflectometer measures the echoes related to its own test signal but likewise to those of the others. These other signals comprise additive noise capable of rendering the use of the measurement impossible. The elimination of this disturbance, which is called “interference noise,” is therefore a necessity.
Sources S0, . . . , Sp-1 transmit the various test signals on the network 20. The reflectometer 21, which corresponds to a source S0, receives a sum Σ of all of these signals filtered by the network 20. Hp1,p2 represents the transfer function of the path taken by the signal between the reflectometers p1 and p2.
The document cited [1] at the end of the specification describes a reflectometry method, which consists in using sequence M-type (or “maximum-length sequence”) pseudo-random sequences as a test signal and in minimising the intercorrelation between the sequences generated by the various reflectometers. The post-processing (discrimination algorithm) next consists in applying matched filtering as in the STDR (Sequence Time Domain Reflectometry) method.
Such a reflectometry method thus described in the cited document [1] has the following disadvantages, in particular:                it does not enable complete cancelling of the contribution of the other reflectometers. Residual noise remains,        it requires the use of an M sequence-type (or LFSR, Logical Feedback Shift Register, of maximum length). In some applications, test signals, such as multicarrier signals, may be necessary due to the constraints of the application in question.        
The aim of the invention is a so-called weighted average distributed reflectometry device and method enabling these disadvantages to be resolved by improving the measuring accuracy.